This invention relates to a waste package of radioactive waste and a method of producing such a waste package of radioactive waste. More particularly, the invention relates to a treatment of concentrated radioactive waste liquid generated from nuclear power plants, etc., and a used ion exchange resin also released from such plants while carrying radioactive substances thereon.
Compaction (volume reduction) and solidification of radioactive wastes generated from nuclear power plants is not only important for securing the space for storage of radioactive wastes within the compounds of power stations but is also a key factor for storage on land which is one of the final disposal methods. Efforts have been made for finding effective means for volume reduction of radioactive waste and a method has been proposed in which a slurry of concentrated waste liquid (basically composed of Na.sub.2 SO.sub.4) and used ion exchange resin, which are the main wastes produced from BWR power plants, is dried and powdered to remove water which occupies a substantial portion of the whole volume of radioactive waste and the powdered material is pelletized. It has been confirmed that this method can realize a volume reduction to approximately 1/8 based on the conventional method in the waste liquid or slurry is directly solidified with cement. However, even this method, though remarkable in its volume reducing effect, has a drawback that it is unable to form a stable solidified body when using a hydraulic solidifying agent such as cement. This is for the reason that the pellets principally composed of Na.sub.2 SO.sub.4 or ion exchange resin swell up by absorbing water contained in the solidifying agent to cause break of the solidified body. As a souution to this problem, a method has been proposed in which an alkali silicate solution is used as solidifying agent and a water absorbing agent is added thereto to make a more stable solidified body of pellets (Japanese Patent Laid-Open 197500/82). Any of the proposed methods, however, involves difficulties in pelletizing the dry powder and also has a problem of high cost due to the necessity of using a drying and powdering apparatus as well as a pelletizing machine.
To avoid these problems, studies are also made on the method in which the dry powder is not pelletized but directly mixed uniformly with a solidifying material and solidified. In this case, plastic, asphalt or inorganic solidifying medium is used as solidifying agent.
For plastic solidification, usually a thermosetting resin is used as solidifying agent, but thermosetting resin becomes unable to fully perform its ability as solidifying agent if even a slight amount of water is mixed therein. This is for the following raason.
When water is brought into the powder-resin mixture in the course of solidification, the hardening promotors (such as cobalt naphthenate) in the thermosetting resin are decomposed to retard hardening of the resin, causing a part of the resin to leave in the state (liquid) it had at the time of addition.
Even if the used ion exchange resin or Na.sub.2 SO.sub.4 is carefully dried, water may not be removed perfectly.
Thus, if the used ion exchange resin or Na.sub.2 SO.sub.4 containing even a slight quantity of water and a thermosetting resin are mixed and solidified, there cannot be obtained a solidified body with high strength. Therefore, the powder dried by a drying means such as thin-film dryer must be placed under a strict moisture control by constantly measuring the moisture content by a neutron moisture meter or other means.
In case of using asphalt, said moisture control becomes unnecessary since the powder of waste material is heated while mixed with asphalt to remove moisture and then solidified. Asphalt, however, because of its thermoplastic nature, has a problem that it is fluidized at 40.degree.-50.degree. C., so that the disposal or storage of asphalt-solidified waste material on land is undesirable.
Solidification by inorganic solidifying agent is preferred for storage and disposal of waste material on land because of good matching of such solidiying agent with soil and rock, and the solidification techniques by use of cement or sodium silicate (water glass) as solidifying agent are studied. Such solidifying agent is mixed with a proper amount of water and powder of waste material to form a solidified block. In this case, the powder of waste material is markedly increased in its contact area with the solidifying material and water, quite different from the case where the powder of waste material is compressed and shaped into pellets. Therefore, if the powder of waste material is chemically reacted with the solidifying agent, the formed solidified body is seriously affected by such chemical reaction. Also, in case the powder of waste material is of the type which is soluble in water, there is the possibility that outside water would penetrate into the solidified body through fine pores in the body and dissolve the waste material in the body, causing leakage of waste material to the outside of the solidified body. This problem is highlighted espccially in the case of dry powder (the main component being Na.sub.2 SO.sub.4) of concentrated BWR waste liquid. For instance, when Na.sub.2 SO.sub.4) powder is solidified with cement, calcium aluminate (3CaO.Al.sub.2 O.sub.3) and calcium hydroxide (Ca(OH).sub.2) in the cement composition are reacted with sodium sulfate (Na.sub. 2 SO.sub.4) to produce ettrigite as shown by the following formula, which causes a volume expansion of the solidified body to break it. ##STR1##
When sodium silicate (water glass) is used as solidifying agent, the reaction of formula (1) doesn't occur and therefore the problem of volume expansion can be evaded, but in case the solidified body is immersed in water, since sodium sulfate is soluble in water, it is hard to perfectly prevent the elution of waste material from the solidified body.
For solving this problem, it needs to turn the soluble sodium sulfate into a water-insoluble state, and as a method thereof, it has been propssed to coat the surface of sodium sulfate with a resin. This method, however, necessitates an extra apparatus for high-speed stirring of the mixture and also has a disadvantage that the volume of waste material to be treated is increased. The similar problems arise in case the dry powder of concentrated PWR waste liquid is solidified.
Use of inorganic solidifying agent for solidifying the dry powder of used ion exchange resin also involves the following problems associated with the properties of ion exchange resin:
(1) The hardening reaction of the solidifying agent is obstructed by the ion exchange groups (mostly SO.sub.3 H) in the ion exchange resin.
(2) The ion exchange resin swells as it absorbs water, causing a reduction of the waste packing rate.
It is possible to evade the problem of (1) by beforehand having the cations adsorbed on the ion exchange groups for inactivating them, but no effective countermeasure is available against the problem of (2)